It's not Stripe or Visa "going poof" that would be an issue it would the underlying banks they use that add risk. Stripe/Visa are not FTX. Payment processors don't use in-flight transactions to pay liabilities (salaries, rent, etc.). They also don't (as far as I'm aware) convert that cash into other assets like bonds.

However, the money may be stored in a money market account i.e short term bonds or even if it's a more normal savings account the bank itself depends on fractional lending and bonds.

I'm be very surprised if any major payment processor hasn't vetted the institutions that hold transaction funds. That said some payroll companies were hit by SVB's collapse but more so on not being able to accept checks than loosing already received funds.